One thought, cant one reason to the difference when you checked be that Firefox is the active application and have focus? And do you maybe have any addons in Firefox? Extensions can give bigger energy impact, you can check that under about:performance. Anyways I think both have low energy impact even if safari happens to be abit lower.
Personally I use Firefox on mac and can many times have 100+ tabs and it works without any problems.
It could also be mention that Firefox now use macos coreanimation that drastically improved Firefox battery performance [1]. So give it a real test again, Firefox is really awesome and needs all support :) We need bigger diversity among webbrowsers, not just chromium based.
[1]
https://mozillagfx.wordpress.com/2019/10/22/dramatically-red...